Third generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technology are widely spread all over the world. High-speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with a radio access technique that is highly competitive in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future. Reduction of cost per bit, increase of service availability, flexible use of frequency bands, simple structure and open interface, proper power consumption of a user equipment (UE), and the like are defined as requirements.
In general, there are one or more cells within the coverage of a base station (BS). One cell may include a plurality of UEs. A UE is generally subjected to a random access procedure to access a network. The random access procedure is performed by the UE for the purposes of initial access, handover, scheduling request, timing alignment, etc.
The random access procedure can be classified into a contention based random access procedure and a non-contention based random access procedure. A greatest difference between the two random access procedures lies in whether a random access preamble is dedicatedly assigned to one UE. In the non-contention based access procedure, since a UE uses only the random access preamble dedicatedly assigned to the UE, contention (or collision) with another UE does not occur. The contention occurs when two or more UEs attempt the random access procedure by using the same random access preamble through the same resource. In the contention based random access procedure, there is a possibility of contention since a random access preamble used by the UEs is randomly selected.
In a wireless communication system, time alignment between a UE and a BS is important so as to minimize interference between users. The random access procedure is performed for uplink alignment. While the random access procedure is performed, the UE is time-aligned according to a time alignment value transmitted from the BS. When uplink alignment is achieved, the UE starts a time alignment timer. If the time alignment timer is running, it is regarded that the UE and the BS are uplink-aligned with each other. If the time alignment timer expires or is not running, it is regarded that the UE and the BS are not aligned with each other. In this case, uplink transmission cannot be achieved except for transmission of the random access preamble.
If the time alignment value is always included in a random access response, upon receiving the random access response, the UE starts or restarts the time alignment timer after applying the time alignment value. Uplink time alignment management is not always required in the random access procedure according to a cell size.
For example, in case of a cell having a significantly large coverage, a distance between the BS and the UE can be significantly far. Thus, a time alignment determined by the BS may differ from a time point of receiving uplink data transmitted by the UE. Accordingly, the uplink time alignment management is necessary. In comparison thereto, in case of a cell having a relatively small coverage such as a femto-cell or an indoor BS, the distance between the BS and the UE is not significantly far. Thus, it may not be necessary to adjust the time alignment during the random access procedure. After the uplink alignment is achieved, the time alignment can be exactly maintained in most of UEs without time correction.
In case of a cell having a small coverage, if the time alignment value is unnecessarily included in the random access response, there is a problem in that radio resources are wasted to transmit the random access response.